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On Thu, 23 Jun 1994, Carl G Heinzl wrote: > Richard, > > I'm now very curious to know the configuration of your cylinders > for such a dive. It sounds like we're talking a minimum of 4 tanks - > two large cylinders at least (?) of bottom gas (were they separate > or connected with a manifold?), one fairly large cylinder of nitrox > (what blend?) plus some amount of O2, perhaps 20-40 cu ft??? The rig has been described in detail in a couple of publications, so I'll just provide a summary here: Yes, 4 cylinders: 2 100cu. ft. cylinders, one of which has trimix, the other nitrox. One 30 cu. ft. pony with trimix, one 20 cu. ft. pony with oxygen. The main trimix cylinder provides enough gas for me to go from 130' down to the working depth (maximum of about 400-425'), spend 10-15 minutes at that depth, and return to 130'. I usually arrive back at 130' with about 500-750 psi in the main trimix tank, and the full 30' in the trimix pony. Through emperical experimentation I have found that the 30' pony provides enough gas to allow a direct but controlled ascent from 400'+ to 130', with a bit to spare. This means I have about 30-40% of my total trimix supply left-over as a buffer at the end of the trimix portion of the dive. This, I feel, is an ample safety margin, since I NEVER dive this rig under a "real" overhead environment (i.e., nothing between me and the surface except water and a few fishes). The nitrox cylinder has a y-valve, and provides me with about 40% more nitrox than I need. I breathe the nitrox as travel-mix from surface to 130', then during the intermediate (130'-30') decompression stops. In some circumstances, I hang an extra nitrox cylinder on the anchor line, just as a backup (i.e., not to be counted on). The nitrox is almost always NOAA I (32% O2). I could use a more oxygen-rich mixture, which would probably get me out of the water faster, but then I would need more trimix for the initial (deepest) decompression stops. Also, in the unlikely event that I have a problem with both the primary & backup trimix supplies, I'd rather have a "less-toxic" nitrox mix to switch to on the ascent (before anyone starts screaming oxygen toxicity at me, I'm only talking about extreme emergencies: the risk of O2 tox on ascent at 250' with NNI is considerable, but if the only other option is drowning....) The oxygen pony is just a backup - I've never needed to use it (but I have done some tests to make sure it provides enough gas for my decompression profiles - which it does, albeit without much in reserve). The primary oxygen supply is surface-supplied from the boat. The pony I carry with me is used only if I can't get back to the boat. > What type of dive profile/deco schedule did you follow? Sometimes I use tables generated by Randy Bohrer, but mostly I use profiles generated with Pro-Planner (10% safety factor). I keep my bottom times on mix-dives to under 15 minutes (which has proven enough time for me to collect all sorts of interesting fish specimens). Total dive times range from 85-120 minutes. If you work out the calculations for breathing rates, you might conclude that I'm not carrying enough gas to complete the dives as described. All I can say is that, either the published calculations are especially conservative, or I'm some kind of freak. In either case, all my mixed-gas dives are very relaxed - if they become unrelaxed, I abort the dive. I'm generally bottom-mix limited, so I end the dive at a predetermined time, or a predetermined primary trimix supply, whichever comes first. > Btw, Alert Diver just had an interesting article (I think it > was Alert Diver, I've been looking at too many articles lately). > > >From memory (apologies if my brain scrambled the message, it has > happened)... one of their points was that by decompressing > on 50% O2, you get many of the benefits of decompressing on pure O2 > (i.e. deco times were pretty close) with a gas that you can use > as deep as 70' and none of the hassles and/or dangers involved > with pure O2. The reason we use oxygen for decompression is to eliminate the nitrogen from the inspired breath. This maximizes the nitrogen gradient accross the alveolar membranes in our lungs, and thus maximizes the rate at which nitrogen molecules leave our blood and enter our exhaled breath. If you reduce this gradient by adding nitrogen to the breathing mixture, you reduce the rate of net-movement of nitrogen molecules from the blood to the exhaled breath. The extent to which this affects over-all nitrogen-elimination depends on many things, including: heart rate, breathing rate, and dive profile (including what gases were breathed when). The decision of whether or not to breathe pure oxygen on the decompression line depends on many things, like degree of inert-gas exposure (i.e., "fast-tissue" dive or "slow-tissue" dive) and degree of experience/discipline of handling and using oxygen. Personally, I like to have a lot of oxygen on hand to deal with DCS problems, should they arise, so I feel comfortable using O2 on my 20' stop. Well, I hope that addressed some of your questions.... Aloha, Rich
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